The present invention relates to an observation device usable with alignment of objects or members and to an aligner for semiconductor circuits manufacturing, which uses the same.
As is well-known, IC (integrated circuits) and LSI (large scale integrated circuits) are manufactured by superposing a number of complicated circuit patterns. The tendency toward higher processing speed and higher density of patterns means that the width of the circuit lines is continuously required to be smaller and smaller, and the accuracy of alignment is required to be higher and higher even to the extent of the order of sub-microns. To meet such requirements, there is being developed an aligner of the step-and-repeat type, which is called a stepper. In a stepper, a pattern of a reticle is projected onto a wafer at a unit or reduced scale. Because of the limit to the design of the projection lens optical system, the projection area is necessarily limited or small so that the entire wafer surface cannot generally be exposed at one shot. Therefore, to cover the entire surface, the pattern is projected on a part of the wafer surface and stepped to the next part and projected, and this is repeated throughout a wafer. With the increase of the size of the wafer, the number of steps required for one wafer increases so that the time required for processing one wafer increases. On the other hand, prior to each projection of the pattern, i.e., exposure of the wafer to the pattern, the reticle and the wafer must have been aligned. So, how to align them is important from the standpoint of alignment accuracy and time taken to achieve alignment. It is known, as an OFF-AXIS alignment, to first correctly place one of them at a predetermined position outside the exposure station and then move it toward the exposure position by a predetermined distance which is assured by a laser interferometer. This type enables a high spaced operation, but involves problems that the alignment cannot be directly confirmed at the exposure station; that it cannot compensate for a non-linear local distortion which may be created in the wafer with experiences of wafer processing; and that the accuracy of the stage movement monitoring may affect the alignment.
There is a so-called TTL type apparatus wherein the wafer is observed through the projection lens at or adjacent the exposure position to align it with the reticle. This type can compensate for the local distortion of the wafer and can avoid the inaccuracy in the wafer stage movement so that a better alignment between the reticle and wafer can be expected.
For the TTL system, a laser beam scanning is known fior the alignment operation. An example thereof is described in a Japanese Laid-open Patent Application No. 54-53562 which has been filed by the Assignee of the present application. FIG. 1 shows a schematical view of the device disclosed therein, for the sake of explanation. A single laser beam from a single laser beam source 1 is split or divided into two beams, which are then directed to the lefthand and righthand objective optical systems 11, thus allowing to detect the displacement or degree of misalignment between the reticle 12 and wafer 13 at two positions. The two position detection allows two kinds of displacement, that is, X and Y direction (translational) displacement and .theta. (rotational) displacement to be corrected, by moving one of the reticle or the wafer relative to the other.
The optical system disclosed in FIG. 1 includes a condenser lens 2 for focusing the laser beam, a polygonal mirror 3, an f-.theta. lens 4 and a beam splitter 5. The laser beam emitted from the laser beam generator 1 is scanningly deflected by the polygonal mirror 3 and is incident on the beam splitter 5 and et seq. The system further includes a field lens 6, a view field splitting prism 25 which is effective also to divide the scanning laser beam into two beams. Because of the dual functions, the prism 25 may be said to be a view field dividing and spatial dividing prism. Each beam is passed through or reflected by a polarization beam splitter 7, a relay lens 8 and a beam splitter 9 and reaches an objective lens 11, by which it is imaged on the objects to scan the same. The system of optical elements extending from a pupil imaging lens 14 to a detector 18 constitutes a photoelectric detection system. The device further includes a chromatic filter 15; a spatial frequency filter 16 for blocking specularly reflected (by the reticle or wafer) beams but allowing scatteredly reflected beam to transmit; an illumination optical system having a condenser lens 17, a light source 19, a condenser lens 20 and a chromatic filter 21; an observation optical system having an erector 22 and an eye piece 23. The function and operation of those elements are explained in detail in the above-identified Patent Application, so that detailed explanations thereof are omitted for the sake of simplicity. In this example, the deflected beam, deflected by the polygonal mirror 3, is divided in its deflection range, by the view field dividing prism 25 which is optically conjugate with the reticle 12 and wafer 13, thus using effectively the quantity of light of the laser beam. The deflection line is transverse to the leading edge 25a of the prism 25. The respective beams divided out by the prism 25 are directed through the respective objective lens 11 to the alignment marks represented by a letter "F", and scan the same, respectively. The alignment scope having the microscopes has an additional important function, i.e., the observation of the alignment marks. The observation is one of the necessary functions, particularly, in monitoring the state of alignment and initial setting of a reticle. For the observation optical system, it is desired that the images are observed in a natural and easy manner.
FIG. 2 shows the image view fields observed through the eye piece 23 in the arrangement of FIG. 1. In FIG. 2, reference numeral 31 depicts the view field dividing line provided by the edge 25a of the view field splitting prism 25; 32, the scanning line of the laser beam; 33, the view field through the righthand side objective lens; and 34, the view field through the lefthand side objective. The alignment mark are represented as the letter "F" for the sake of simplicity of explanation. As shown in FIG. 2, the lefthand side alignment mark on the reticle 12 and the righthand side mark thereon are observed at the right and left view fields, respectively, through the microscope as erected images. The alignment marks play important roles in manufacturing semiconductor circuits, but do not provide any actual circuit patterns. So, after the wafer has been completely processed, the part thereof having the alignment marks are the non-usable areas. For this reason, the area occupied by an alignment mark is desirably as small as possible, so as to provide a better yield.
FIG. 3 shows an example of a reticle or mask (hereinafter called simply a reticle). If the alignment marks are provided on the scribe lines between adjacent chips 101, they do not require any particular space, so that the above-described problem is solved. Since the scanning laser beam runs in the direction connecting the alignment marks (this direction is called longitudinal direction in this specification), two alignment marks are arranged along this direction, that is, along and within a scribe line which is near the center of the reticle. The scribe line near the center, used for this purpose, is desirable since highly accurate alignment can be obtained.
However, in the case of a so-called stepper type exposure and alignment device, inter alia, in the reduction stepper, it is possible that one reticle, at its entity, corresponds to one chip so that there are scribe lines only at the marginal area, that is, no scribe lines are near the center which would be better to accommodate the alignment marks as explained above.
FIG. 4 shows a reticle having a pattern of only one chip, wherein the possible alignment marks are shown by reference numerals 102 and 103. Assuming that the scanning beams run from lefthand to righthand sides, for example, it is contemplated that a couple of alignment marks are arranged as indicated by reference numeral 102 or otherwise by reference numeral 103. In the latter case, that is, the case of marks 103, each of the alignment mark areas extends across the circuit pattern area, so that they require wider scribe lines. This is a serious problem since the yield is considerably decreased. In the former case, that is, the case of the alignment marks 102, such control is not so severely required, but the marks 102 are only at the upper side (in the FIG. 4), so that the misalignment is larger at the bottom side.
Those problems can be solved by the alignment marks 104 shown in FIG. 5, i.e., by placing the alignment marks near the center of the reticle and extending the alignment marks in the perpendicular direction i.e., the direction perpendicular to the longitudinal direction which is the direction connecting the two alignment marks, and correspondingly scanning the laser beam in the perpendicular direction.
The perpendicular scanning of the laser beam can be obtained in FIG. 1 arrangement by rotating the systems 1, 2 and 3 including the laser beam system parts and moving a wider laser beam along the edge 25a of the prism 25. This is the separation of the single laser beam along the direction of beam deflection, as contrasted to the FIG. 1 arrangement, as it is, wherein the beam is separated by the edge extending perpendicularly to the direction of beam deflection. Therefore, such modification of FIG. 1 arrangement is not advantageous in that the quantity of light is decreased to one half since the beam is divided along the deflection direction, and also in that noise light is always produced by the edge 25a of the prism 25. It is possible to simply insert a 90.degree. image rotator at a suitable position after the prism 25 to convert a longitudinal direction scan to a perpendicular direction scan. If this is done, however, the reflected beam is also rotated by 90.degree. so that the alignment mark (F) is rotated by 90.degree., thus disabling the natural and easy observation.